Ion-stabilized electron induction accelerator



March 22, 1960 FINKELSTEIN ION-STABILIZED ELECTRON INDUCTION ACCELERATOR 3 Sheets-Sheet 1 Filed April 28., 1958 V 4 i, I "r/4 I I I I I I 44 I l I INVENTOR. DAVID FINKELSTEIN BY W 4 Mrch 22, 1960 D. FIN KELST EIN 2,929,951

ION-STABILIZED ELECTRON INDUCTION ACCELERATOR Filed April 28; 1958 3 Sheets-Sheet 2 Z'7- INVENTOR.

" DAVID FINKELSTEIN March 22, 1960 D. FINKELSTEIN 3 Sheets-Sheet 3 mmm INVENTOR.

DAVID FINKELSTEIN period.

ION-STABILIZED ELECTRON INDUCTION ACQ'JELERATOR David Finkeistein, Hohoken, NJ assignor to the United States of America as representedhy the United States Atomic Energy Commission Application April 28, 1958, Serial No. 731,555

7 Claims. c1. s13 .-1s7

United States Patnt It had been proposedby Budker in the CERNSymposium on High Energy Acceleratorsfp. 68, 1956, that the self-focusing magnetic field of an ion-stabilized, relativistic electron beam (discussed by Bennett in the Physical Review, volume 9 8,.pj1584, 1955), be utilized to accelerate ions to veryhigh energies. The proposal was that such a beam might be developed from a plasma by applying.

thereto a varying magnetic field similar to the magnetic field of a betatron. The following two conditions are char? acteristic of a betatron magnetic field: (1) the magneticfield fiux density at the particle orbit equals /2 the average flux-density within the orbit; and (2) the flux density' as a function of the distance of the aids from the orbit varies as the nth power of the ratio of theorbit radius to the distance, where n 'is greater than 0 and less than 1.

By the presentinvention there is provided method and apparatus for establishing anion-stabilized, self-focusing relativistic electron beam from a plasma. An especially constructed cavity having a highly conductive, non-magnetic surface is a feature .of the invention. The cavity is a figure of revolution symmetrical 'with respect to, both an axis and a med'ianplane The axial length of the cavity is defined as thedistance between the two planes parallel to the median plane and tangent to the cavity' at its endsand is approximately equal to" the diameter thereof at the median plane. At a certain distance from the median plane, the diameter of the cavity is less than at the median plane and the radius of the cavity over the atrial length first decreases smoothly and then increases smoothly. A magnetic field satisfying the betatron conditions is produced within the cavity by currents flowing in the cavity surface. This magnetic field establishes the beam by induction from a plasma in theflcavity, An induction acceleratorofthe class in which an ion-stabilized,

self foc'using relativistic electron stream is established from a plasma in a-"cavity by amagnetic field produced therein by currents flowing in the surface of the cavity,

.said magnetic field having the betatron characteristics,

is defined as a megatron.

In a preferred embodimentof the invention, the espec'ially constructed accelerator cavity is within and forms a part of a single turn coil having no core. 1 The plasmais injected into the cavity by a plasma gun. Thereafter a I bank ofcapacitors" is discharged across the terminals of the coil causing current to flowfiin'the coila'nd this produces the requisite magnetic field. As the discharge of the capacitors is very rapid, of the'order of microseconds,

substantially all the current flows in the innersurface of the coil as: a result of theskin' effect. After the electron beam has been established, the terminals of the coil are short-circuited. in order 'to retainthe. beam for a aitentecl Mar. .22, teen It is, therefore, an object of the present invention to provide a me'gatron. Another object-of the present invention is to provide a single turninduction accelerator. Still another object of the present invention is to provide a single turn induction accelerator which has an especially constructed cavity. Afurther objectof the present invention'is to provide a single turn induction accelerator which has an especially constructed cavity in which there is produced a magnetic field having the betatron characteristics. An additional object of the present invention is-toprovide a single turn induction accelerator which has an especially constructed cavity in which a magnetic field having the betatron characteristics produced by curent flowing in the surface of the cavity establishes an ion-stabilized, selffocusing, relativistic electron beam in the cavity from a plasma therein.

The study of the physics of a composite neutralized beam of ions and electrons indicates that it will be magnetically self-focusing if the velocities of the ions and electrons in the beam direction are different. Evidently, if the ions and electrons move in opposite directions in the beam, it 'will be self-focusing. Extensive, theoretical treatments of a composite beam of ions. and electrons are contained in the Budker and Bennett articles referred to already. i

Although an electron beam will be produced in the megatron whatever type of plasma is injected therein, it is preferable for development of highintensity neutron beams that the plasma be developed fromrlow-at omicnumber elements, and preferably deuterium, in order that fusion reactions may be obtained. A plasma gun suitable for use as an injector for the megatron is described in Scientific American, volume 197, October 1957. This plasma gun incorporates two parallel conductors of titaniam in which deuterium has been absorbed. A small bank of capacitors is electrically connected to the terminals of the plasma gun through a conventional switch. The capacitor bank is charged in a conventional manner and, upon discharge of the capacitors across'the terminals of the plasma gun,'an arc is developed across the open end of the conductors, establishing a plasma there. Magnetic forces associated with the plasma propel the plasma rapidly outward from the plasma gun.

The invention, both as to its organization and method of operation, together with other objects and advantages ingdrawings in which:

Figure 1 is aiperspective, partially cutaway, of an idealized torus in accordance with the invention and is used to illustrate a method by which the shape of the cavity may be determined.

Figure 2 represents'a curve, derived by calculation, for a megatron cavity configuration.

tron. I

Figure 4 is a diagram showing the typical voltage and current wave forms of the megatron coil.

Figure 5 is a diagrammatic perspective, partially cut away, of the megat'ron and associated'electrical components to show the interior arrangement.

Figure 6' is a perspective view of the cavity envelope of Figure 5 showing conductors forming an electrostatic shield for the cavity plasma. V

Figure 7 is a perspective view, partly cut away, of a plasma switch foruse in rapidly energizing the" coilcircuit of the megatron. I 1

Figure 8 is a partly diagrammatic axial section of. a Y shaped plasma gun and includes the necessary electrical connections.

Figure 9 is an end view of Figure 8.,

Figure 3 is a schematiccircuit diagram of the mega- Figure is a partially diagrammatic axial cross section of a coaxial plasma gun.

Figure 11 is a cross sectional view of a coaxial plasma gun showing the manner of mounting capacitors therewith in a common housing. 7

Figure 12 is a diagrammatic perspective, partly cut away, of some of the essential components of a megatron to showa particular mode of construction of the coil bearing wall and electrical connections.

Referring now to Figures 1 and 2, the nature of the especially constructed cavity of the present invention will be understood through consideration of a technique that may be.used for determining its contour. The circular lines 10, 12 and 14 (Figure 1), disposed coaxially about an axis 16in parallel planes above a median plane defined by the axes 18, 20, represent idealized coils. Similarly, lines 26, 24and 22 are disposed below the median plane defined by axes 18, 20 in mirror-image relation to 10, 12 and 14 respectively. Coils 10 and 26, 12 and 24 and 14 and 22 are thus essentially identical and are to be energized as pairs.

The magnetic field of these idealized coils may be represented by:

total 1O-26+ 12-24 12-24+ 14-22 1442 Where B10 25, B12, 24 and B14 22 are the magnetic fields of idealized coil pairs 10-26, 1224 and 14-42, respectively, when each coil pair carries unit current and a and u are numerical multipliers which measure the currents in coil pairs 12-24 and i k-22 relative to coil pair 1026. [The values of a 'and oz are selected so that the field, B within the orbit has the requisite betatron characteristics as defined above, and so that all field lines enclose each of the coil pairs. An exemplary curve 28 calculated according to this technique is shown in Figure 2, where Z is the distance along the axis 16 from the median plane of axes 18, 20 and R is the distance from the axis 16 in the radial direction along one of the axis 18, 20, both in the same arbitrary units. The value of n in the second condition for the betatron field, as defined before, calculated for Figure 2 equals 0.67 at R=l and Z=0. The torus 30 is generated by revolution of the curve 28 about the axis 16. The pairs of idealized coils 10-26, 1224 and 14-22 within curve 28 are replaced by a highly conducting, nonmagnetic material which has the contour of torus 34 such as beryllium-copper, and this structure is suitable for the megatron. It will be readily apparent that innumerable curves analogous to curve 28 can be obtained for a megatron. Inasmuch as it is the magnetic field in the cavity 32 near the median plane defined by axes 18 and 20 which is important, the cross section of the megatron coil, as it bounds the cavity 32 (between lines 34 and 36 of Figure 2 representing corresponding planes in Figure 1), is made to conform to curve 28 of Figure 2. The outer periphery of the torus 30 has little effect on the magnetic field within the cavity. Accordingly, the outer periphery may take any convenient shape, such as a cylindrical surface.

In the practice of the present invention, a bank of capacitors is rapidly discharged across the terminals of the single turn coil. The coil is preferably constructed of a structurally supported high-conductivity, non-magnetic material. Currents flowing in the surface of the coil adjacent the cavity as result of the skin efiect establish the special magnetic field. In order to maintain the magnetic field constant for an extended period within the cavity formed by the single turn coil, the coil is short circuited when the current therein has reached its maximum value following the discharge of the capacitor bank across its terminals. Simultaneously with or prior to the discharge of the capacitor bank, a plasma is injected into the cavity. As the field in the cavity builds up, a beam of electrons having relativistic energies is produced from the plasma electrons by induction.

It is necessary that the amount of plasma be sufiicient to provide the desired beam current and yet small enough so that the self-magnetic field of the beam can be neglected. If the amount of plasma is insufficient, there will be an insufiicient number of electrons to form a high current beam. Furthermore, when this current is too large, the self-produced magnetic field of the excessive current causes the betatron conditions to be violated. The excess current causes the guide field to be altered by the local beam magnetic field sufficiently to perturb its betatron characteristics.

There is preferably incorporated in the present invention as a part thereof a vacuum plasma switch of novel design. This switch has very low resistance and inductance and therefore permits the necessary rapid discharge of the capacitor bank across the terminals of the megatron coil. In this new switch, a source of plasma injects a highly conducting plasma into a region between the terminals of the switch, and the circuit is closed through the plasma. Thus, this plasma switch afiords rapid switching action for large currents.

The plasma switch can be closed rapidly because the current path through it is established by firing a plasma between its terminals, rather than mechanically bringing two terminals together. The switch has low resistance because the current can be spread over a greater area than in a conventional switch. The switch has a high breakdown potential because of the vacuum gap. The switch has low inductance because the current path-the inductance of a circuit is proportional to the length of the current pathcan be made very short as the switch contacts can be placed very close to the terminals to be connected by the switch.

The capacitor bank must be discharged rapidly across the megatron coil terminals. In order that this occur, the capacitor bank should have low inductance and small power loss and be fast acting. In the practice of the present invention, conventional ignitrons can at times be used for switching if the size of the megatron coil is sufficient to give a relatively large inductance. For others of less inductance, it is preferable to utilize the special plasma switch in which a region between terminals of the switch is rapidly filled with plasma from a plasma source. As the plasma is highly conductive, the switch rapidly discharges thecapacitor bank with little loss.

By referring to Figures 3 and 4, which illustrate the electricalcircuit diagram and wave forms for an idealized megatron, respectively, the electrical operation of its energization will be explained. The megatron coil 38 is connected to a capacitor 40 via switch 42. After capacitor 32 has been charged by a source (not shown) and switch 42 is closed, the coil 38 voltage wave form V rises from point P at time T to point P with a risetime of T,. The idealized voltage V and current I wave forms are shown by the dotted lines. At time T/4, shorting switch 44 is closed to short the coil 38, and the current I in coil 38 remains essentiallyeonstant along line 46 for a relatively long time.

The construction of the megatron and its essential electrical connections are shown diagrammatically in Figures 5 and 6. The megatron coil 48 has two half sections 50 and 52. For purposes of illustration, the outer contour 56 of coil 48 is shown to be cylindrical. However, as described above, it is the inner contour 59 which primarily determines the magnetic field configuration within the cavity 60. Coil 48 is preferably made of a highly conductive, non-magnetic material such as a beryllium-copper 10 alloy (suggested by 'R. Waniek for a high-current, high-magnetic field coil) in order that the magnetic field (represented by the line 62) in cavity 60 may be established by the current (represented by the line 64) flowing on the inner surface of coil 48 with a minimum of power loss. The current in coil 48 is produced by the discharge of the capacitor banks 66 and.

The coil 48 is supported within chamber. 98 f vac-.

uum housing 100 which is evacuated via tube 102 by a vacuum pump, not shown. A-plasma gun 104 is mounted on the base of housing 100- in such a manner that plasma. ejected thereby is injected into cavity 60. of coil 48. Plasma gun 104 is energized by discharge of acondenser 1%.-through a switch 108.

Within cavity 60 the plasma is kept away from the sur face 59 of coil 48 by a glass envelope 110. Glass envelope 110 is mounted on the base ofhousing 100 and is separately evacuated via a port 112 in. the base of housing 100. Plasma gun 104 injects theplasma into the envelope through the port 114. Insidethe envelope, there is an electrostatic shield 116.- (illustrated. in Figure 6) on the interior surface thereof to eliminate any per-. turbation of the plasma caused by the electrostatic field of the coil gaps. In order that no current flows in the electrostatic shield, the conductors 112 thereof are joined at one end at juncture 120 and grounded by conductor 122 and left open at the other. In one embodiment of the megatron there are lines of a conductive material in.-

scribed on the inner surface of envelope 110. The electrostatic shield is grounded in order that any charge that accumulates thereon does not build up.

With reference to the megatron of Figure 5, its opera I tion is as follows: Chamber 98 and an envelope 110 are evacuated by vacuum pumps, not shown. Capacitors 66 and 68 are charged in a conventional manner ralityof plasma guns mounted on housing 100. The

the magnetic field in cavity 60 begins to buildup as soon as the plasma arrives. The magnetic. field acts upon the plasma injected into cavity. 60 by induction in such a manner that an orbital. beam 100 of relativistic electhe order of about 1%. If conventional targets are present in the cavity near the shrinking beam, there may be. produced, as. a result of beam impingement, energetic gamma rays and electronuclear excitations. When; no target is present,.one may-obtain a large reductionin the crosssectional area ofthe electron beam as a result of the pinch effect. The. plasma density within the beam then increases, and the self-magnetic field of the beam also increases since the latter varies approximately as the ratio of the electron beam current to the radius of thefbeam. As a result of the shrinkage of'thecro'sssection of the beam, the reactions'just mentioned may occur within the beam withouta separate target.

Ifthere 'isfany hydrotnagnetie instability of the beam, is possible to *stabilize it by an azimuthal magnetic iie liwithin; and adjacent to the beams Such a magnetic.

.plasma switch, the deposition can be readily as by scraping.

plasma gunsinject plasma into gaps 70 and 72 to provide a highly conductive path thereby shorting terminals and 82 and and 92.

In a particular embodiment of a megatron in accord ance with the present invention, a special plasma switch referred to above and described in detail below with ref-. erence to Figure 7 is used for switches 78 and 88. The switch includes a body 124, terminals 126 and 128 mounted thereon, a chamber 130 defined by the body 124 and terminals 126 and 128 and a plasma gun (not shown) mounted on terminal 126 at opening 132. Body 124 is of an electrically insulating material which, for low repetition rates, may beLucite. Terminals 126'and 128 are attached to body' 124 in recesses 132 and 134 by screws 135 and are vacuum sealed by vacuum rings 136 and 138, respectively. A projection 140 on terminal 128 projects into chamber 130 close to terminal 126. Chamber 130 is evacuated via opening 142 in body 124 by a conventionalvacuum pump, not shown.

The operation of the plasma switch of Figure 7 when incorporated in an electrical circuit, particularly a megatron, to connect two conductors is as'follows; ,Each terminal 126 and 128 is connected to an alternate con-. ductor as by bolting or welding to make good electrical contact therewith; Chamber 130 is evacuated and plas: ma is: inje'cteditherein at-opening 132 inthe direction of the arrow 143. The plasma establishes a highly conductive path between terminals 126 and128, particularly between the adjacent areas of terminal 126 and projection 140, and this makes electrical switch connection between the conductors.

In the event there is metallic deposition on the insula-v tion surface of chamber 30, it isdesirable to make it ribbed as by ribs 144. Ribs 144 present a largeearea for metallic deposition and any thereon will be thin. If ever metallic deposition on the insulation surface of chamber 30. should impair the switching action of the removed There is nothing critical about having a projection 140, mounting the plasma gun on terminal 126, orhaving only one plasma gun, or forming the plasma switch in the particular manner shown. Rather, the essential features of a plasma switch are terminals, an evacuated region between the terminals, at least one plasma gun to provide a highly conductive path between the terminals everywhere they have adjacent areas, means for operating the guns such as a capacitor and a switch and means for charging said capacitor.

A plasma gun suitable for application with my invenand the specification and drawings thereof are incorporated by reference herein.

Another plasmagunsuitable for application with my invention, both as the injector for the cavity and for the plasma switch, is shown in Figures 8 and 9. This is a Yashaped gundesigned by WinstonH. Bostick. A purpose of the gun is to produce a relatively pure deuterium plasma from: a plasma containing both titanium and deuterium. The relatively pure deuterium plasma is obtained from two more streams of plasma from several plasma guns of the nature of that disclosed in Patent No. 2,900, 538, and the. ScientificAmerican'article referred toabove; The plasmas are caused to follow a lengthy path within an insulator: housing thus providing an enlarged surface upon which. the titanium can deposit. Generally, the

assent 'ed on an insulator housing'152 having twoadjacenttubes 15.4 and 156. An electric field isestab lished between terminals 158 and 160. Plasma guns 148 and150'in ject plasma into tubes 154 and 156, respectively; Magnetic forces cause a plasma to be ejected through orifices 162 and 164 in insulation sheet 166. Therefrom the plasma indicated by arrow 170 exits via tube 168.

' More specifically, the construction of the Y-shaped plasma gun of Figures ,8 and 9 is as follows: Plasma guns 148 and 150 are sealed in radial arms 172 and 174 of non-conductive tubes 154 and 156 (of glass, for example), respectively. Tubes 154 and 156 are separated by spacer 176. Plasma guns 148 and 150 utilize titanium absorbers and deuterium atoms. Their plasmas, represented by. the arrows 178 and 180, are injected radially toward the axes of tubes 154 and respectiyely Terminals 158 and 160 are sealed in adjacent ends of tubes 154 and 156. Electrode 158 is connected via conductor 182 to capacitor 184. Capacitor 184 is connected via switch 186 and conductor 188 to electrode 160. The Y-shaped plasma gun (tubes 154 and 156) is evacuated in a conventional manner, capacitor 184 is charged by a conventional means not shown, and switch 186 is closed. Capacitors 190 and 192 are charged by a means not shown. Plasma guns 148 and 150 are fired simultaneously by closing switches 194 and 196 at the same time. The plasmas 178 and 180 provide a conducting path for electrical discharge between electrodes 158 and 160, and as a result plasma is forcibly ejected from tubes 154 and 156 through openings 162 and 164, respectively, in insulation sheet 166 which seals the adjacent. ends of tubes 154 and 156. The plasma streams which enter tube 168, sealed to sheet 166 around openings 162 and 164, and exits from tube 168 as plasma 170, is a purer deuterium plasma than the plasmas 178 and 180. The plasmas 178 and 180 contain both titanium and deuterium. A considerable proportion of the titanium deposits on the walls of tubes 154 and 156 and as a consequence plasma 170 consists of relatively pure deuterium.

Another type of plasma gun which I have found suitable for application with the megatron as a plasma injector for the aforedescribed plasma switch and for the megatron cavity and gaps is shown in Figures and 11. It is known as a coaxial plasma gun. Referring first to Figure 10, a simple form of the coaxial plasma gun comprises a rigid conductor 200 and, coaxial therewith, a conductive, cylindrical, cup-like terminal 202 having a round flat base 205 and a cylindrical wall surrounding one end of conductor 200. Conductor 200 is mounted within the outer terminal 202 in a rigid insulating block 204 which maintains the spacing. Terminal 202 has an opening 206 in its base 205 opposite the end of conductor 200, and the opening continues through the insulator 204 to the end of conductor 200. When a capacitor, not shown, is discharged across conductor 200 and terminal 202, an electrical discharge is initiated between conductor 200 and the surface of orifice 206. The plasma formed near the orifice 206 is ejected therefrom by the pinch effect. The plasma comprises atoms mainly of the conductor 200 and partially of the base 205. By making the tip 208 of conductor 200 of difierent materials, such as titanium with absorbed deuterium, various plasmas may be obtained.

Another embodiment of the coaxial plasma gun is shown in Figure 11. The gun incorporates capacitors 210 and 212 and a coaxial plasma source 214 in a common housing 216. Housing 216 comprises a container 218 mounted in vacuum sealed relation on housing base 220 by screws 222 and vacuum ring 224 and is grounded. Capacitor 210 comprises a low voltage plate 226 in the form of a cylindrical sleeve mounted on base 220, a, dielectric 223 and high voltage plate 230. Dielectric 228 is a cylindrical sleeve of a high dielectric material such as barium-titanate about low voltage plate 226. High 8 voltage terminal 230 is a cylindrical sleeve about a pottion of dielectric 228. Plate 230 is axially shorterthan plate 226 to avoid any undesirable end effects. Similarly, capacitor 212 comprises low voltage plate 232, dielectric 234 and high voltage plate 236.

High voltage plates 230 and 236 are connected by connector 238 with which high voltage lead 240 is in contact by contactor 242, a spring loop. High voltage lead 240 comprises a high voltage conductor 244 and insulator sleeve 246 and passes into housing 218 in vacuum sealed relation. Coaxial plasma gun base terminal 248 is mounted on connector 238 and is separated in insulated relation from coaxial plasma gun sleeve terminal 250 by insulator plate 252 in which opening 254 exposes partly base terminal 248. Sleeve terminal 250 and housing base 220 have a common opening 256. v

The operation of the coaxial plasma g'un of Figure ll is as follows: Capacitors 210 and 212 are charged from a high voltage source, not shown, by high voltage lead 240. Electrical discharge occurs between gun base terminal 248 and sleeve terminal 250 via opening 254. The discharge may be initiated by injection of an ionized gas into sleeve terminal 250 at opening 256 in a conventional manner. Plasma forms adjacent opening 254, and magnetic forces due to the pinch effect propel it from the coaxial plasma gun in the direction of arrow 260.

'During the operation of the megatron, the varying magnetic field will create extremely large forces on the coil thereof. It is important, therefore, that the construction of the megatron be such that these forces are accommodated so as to prevent movement of the coil struc-' ture. As extremely high currents must be established rapidly in the megatron coil, it is important that the capacitor banks be rapidly discharged through low inductive paths across the megatron coil gaps.

Figure 12 illustrates another embodiment of this invention showing a megatron coil 300, coil bearing wall 302 and electrical components including capacitor banks 304 and 306 electrically connected to coil 300 via transmission lines 308 and 310, respectively. Although the megatron coil 300 may be fabricated in any number of sections, it is convenient to fabricate it in two sections indicated as 312 and 314 delimiting the cavity 316 in the manner described with respect to Figures 1 and 5. Coil sections 312 and 314 may each be of unitary construction, but again for convenience of fabrication, they are each made of multiple layers or lamina, four in the embodiment illustrated. The matching faces of the layers are finely machined and lapped at the contact surfaces. A bearing wall structure 302 peripherally surrounds the megatron coil 300. Bearing wall 302 is spaced from coil 300 by a, plurality of insulator support cylinders 318. Coil sections 312 and 314 are separated for support from one another by a plurality of insulator cylinders 320. Support cylinders 318 and 320 accommodate the forces resulting from build-up and decay of the magnetic field in cavity 316 of coil 300.

Although the bearing wall 302 may be of any suitable construction, in the embodiment of Figure 12, it comprises four layers, 322, 324, 326 and 328 of the same material as coil 300. For this purpose, berylliumcopper 10 has Proven to be satisfactory, both as to its electrical and structural properties. Layers 322, 324, 326 and 328 are annular rings spaced from each other by annular insulating gaskets 330, 332 and 334. The particular construction of bearing wall 302 described above makes it possible to energize coil 300 by a transmission-line technique.

Low-inductance capacitor banks 304 and 306, of conventional design, are connected via plasma switches 336 and 338 (as described above) and transmission lines 308 and 310, respectively, to bearing wall 302. In greater detail, these connections are made as follows: Capaci: tor bank 304 is connected via conductors 340 and 342 and plasma switch 336' to a transmission line 308 of mass.

special construction.

and 346; respectively; of coppeig foreitamplet The sheets are flat; and trapezoidal-irish'a'pe taperingeinward; toward wall 302; The sheets are separated by a'con' giuent insulatorJ348; Transmission 'lines 308 and 310 are preferably of trapezoidal shape so that if fdesireda plurality of transmission lines and associated capacitor banks may readily-be disposed: circumferentially about bearing wall 302. The nar'row'end--of transmission line 388 is inserted betweenthe lowerpairof bearing wall layers 326-and 328 so that lo wer sheet--3,46- makes electrical contactiwith layer 328:: A'n'electrical jumper 350 connects layer" 328 to the lowest layer 3520f coil sec tion 314,- and an electrical jumper 354 (shown in phantom outline) connects-bearing wall layeri326-to nextto the lowestlayer356 of -coil section-312. Similarly; capacitor bank 306 is connectedvia plasma switch 338 and conductors 358 and 360- to a transmission line-310 of similar construction toj transmission-line 308. The latter comprises upper and" town conductive sheets 362 and 364; respectively Upper sheet-.362 is-separated from-.lowerf sheet by;an insulator 366.v Transmission line 310 is inserted between the upper pair of bearing-wall layers 322 and 324- sothat the upper sheet 362- makes electrical contact with laycr32-2- and the-lower sheet 368 makes electrical contact with layer 324. Electrical jumper 368 connects the uppermost wall'layer 324 with the uppermost layer 370 of coil section 312-; and'electrical jumper 372 makes electrical contact'betweenwall layer324 and layer 375, the next front-the topJlayer of coil section 314. f v

I Themagnetic fieldi'n: the cavity 316 is'established as follows: Capacitor banks; 304 and 306 are charged by a conventional means notshownt, Plasma switches 336 and 338 are firedsimultaneously, and a series electrical conductive discharge path is" established. for capacitor banks 304 and 306 through coil- 300. Current flows from capacitor bank 306 viaconductor 360 through upper sheet 362 of transmission line 310 to bearingwall layer- 322 and by jumper 368'to-coil section 312. The current flows from.coi1 section 31-2'via jumper 354 to hearing wall layer 326. Thence the current flows through upper sheet 344 of"tra' ns f i ssionl line 308"via conductor 340 to capac'ito'nba-hls 304. The current return path to capacitor bank 306 continues fromcapacitor b anki304 via conductor 342 and plasmaswitch 336 through lower sheet 346 of transmission line 308 to hearing wall layer 328. Jumper 350 makes the electrical connection from layer 328 to' coil section 314 at its lowermost layer 352. Gurrent then. p-assesfrom section 314 viajumper 372 to bearing wall layer 324 and therefrom" via lower sheet 364 of transmission line 310 to plasma switch 338. Plasma switch 338 and conductor 358 close the return current path to capacitor bank 306. The current thus established flowing in the surface of cavity 316 of coil 300 produces the megatron magnetic field havingthe betatron characteristicsin cavity 316:? As descaibed,

before, the-magnetic field through induction establishes an ion-stabilized relativistic electron beam from a plasma,

all as aforedescribed.

'In order that the magnetic separating bearing wall 302 and coil 300 be relatively large in area compared 'to the area of the opening 378 at the end of cavity 316. Opening 378 is at the nar- I field in the cavity 316 be rapidly established,'it is necessary that the annulus 376.

' ance with Figure 12. areas follows:

field. 1 a

Some parameters of -amegat-roncOnstrucIedin accQrdQ Therefore, ina megatr'on; a toroid'abb'eamof: electraits of very highcurrentdensity and current=iscone strained by a relatively strong external magnetic; field.. and maintained neutral. in total charge by ions'.there-i within. The positive ions of the plasma 1 within the-cavity neutralize the repulsive forces among the beam-electrons, and the'magnetic field associatedwith the beamcurrent causes tremendous forces by the pinch effect to compress. the electrons of the beam'and ions trapped therewithin. An accelerator incorporating a megatron with a three meter electron orbit radius may possibly beconstructed to hold Bev. protonsin the orbit and the protonswill always be constrained by the self-focusing fields:

While the invention has been disclosed with respect to. certain preferred embodiments, it will be apparentto those skilled in the art that numerous variations; and modifications may be made withini'the spirit and scope. of the'invention, andthusit is not .intended to'lirnit tlte inventionexce'pt as-defined'in the following claims.

What is claimed is: r

1. Apparatus including acavity formed by a single! turn coil; means for causing a varyingcurrent to' flow in the surface of said coil surounding said cavity whereby a magnetic field having the betaton characteristics is established insaid cavity, a plasma source" for said cavity; an insulator envelope in said cavity'insulating' said sur face from said plasma, an electrostatic shield on. the

plasma side of said envelope whereby anion-stabilized;

self-focusing, relativistic electron: beam is establisheddn said cavity, and switch means to terminaterthe variation of said current to cause said current to flow invariably in said surface thereby maintaining saichbeam:v for: a relatively long period.-

2. Apparatus including a cavity formed by? a single ing a plasma into said cavity; an insulatonenvelope in V said cavity insulating said cavity from said plasma, an electrostatic shield on the plasma side of said envelope, means for causing a varying current to flow' in said coil surface, said means including a capacitor means whereby a. magnetic field having the betatron characteristics is established in said cavity, whereby an ion-stabilized, selffocusing, relativistic-electron beam is established in said cavity and means for shorting said terminals whereby said electron beam is maintained in said cavity for a relatively long period.

3. The apparatus of claim 2 in which the means for shorting said terminals consists of at least one plasma gun.

4. Apparatus including a cavity formed by a singleturn coil, said coil having a plurality of pairs of terminals, capacitor means, a plasma switch between said capacitor means and said coil terminals to cause a vary 11 fng current to flow, in' said coil, said coil being soarranged and adapted that said currentestablishes a mag-j netic field having the betatron characteristics in said cavity; said plasma switch including at least a pair of electrical terminals, said'terminals bounding a region, means forvevacuating said region, and plasma gun means for' establishing a plasma in said region between said terminals, at least one additional plasma gun for injecting plasmainto said cavity, an insulator enevelope in said cavity to insulate said plasma from said cavity surface whereby an ion-stabilized, self-focusing, relativistic electron beam is established by induction in said cavity from said plasma and plasma gun means for shorting said coil terminals to maintain said beam in said cavity for a relatively long period.

5. Apparatus including a cavity formed by a singleturn coil,.said coil having a plurality of pairs of terminals, capacitor means, a plasma switch between said capacitor means and saidcoil terminals to cause a varying current to flow in said coil, said coil and current being so arranged and so adapted that said current establishes a magnetic field having the betatron characteristics in said cavity, said plasma switch including at least a pair of electrodes, said electrodes bounding a region, means for evacuating said region, and plasma gun means for establishing a plasma in said region adjacent said electrodes, at least one additional plasma gun for injecting plasma into said cavity, said additional plasma gun being a coaxial plasma source including a rodlike terminal, an Outer terminal adjacent said rodlike terminal and insulated therefrom, said outer terminal having an opening therein coaxial with said rodlike terminal and communiea'ting with said. terminal whereby electrical discharge from said rodlike terminal to the peripheral sur face of said opening causes a plasma to be propelled from said coaxial gun by the pitch effect, an insulator envelope in said cavity to insulate said plasma from said coil surface whereby an'ion-stabilized, self-focusing, relativisticelectron beam is established by induction in said cavity from said plasma and plasma gun means for shorting said coil terminals to maintain said beam in said cavity for a relatively long period.

6. Apparatus including a cavity formed by a singleturn coil, a plurality of capacitor banks, at least one plasma switch and one transmission line operatively associated with each of said capacitor banks between said capacitor banks and said coil, an electrically conductive bearing wall for said coil, said bearing wall being electrically connected to said coil so that discharge of said capacitor banks causes a varying current to flow in the surface of said coil about said cavity, said coil .being so arranged and adapted that said current flow establishes a magnetic field having the betatron characteristics in said cavity, means for injecting plasma into said cavity whereby an ion-stabilized, self-focusing relativistic-electron beam is established in said cavity fromsaid plasma by induction.

7; A megatron including a cavity formed by a singleturn coil, said coil 'comprisingz'first and second coil sections; said first coil section having first and second terminals; said second coil section having third and fourth terminals; said coil sections being separated by first insulator supports; an electrically conductive bearing wall for said coil; said bearing wall comprising first, second, third and fourth layers and being spaced from said coil by second insulator supports; first and second capacitor banks; first and second plasma switches; first and second transmission lines; said first transmission line having a first upper sheet and a first lower sheet; said second transmission line having a second upper sheet and a second lower sheet; first, sec'o'nd third and fourth electrical jumpers; said first upper sheet being connected to said first layer; said first layer being connected by said first jumper to said first terminal of said first coil section; said second terminal of said second coil section being connected via said second jumper to said second layer; said second layer being connected to said first lower sheet; said first lower sheet being connected to said first upper sheet by said first capacitor bank and said first plasma switch in series; said second upper sheet being connected to said third layer, said third layer being connected to said first coil section by said third jumper; said second coil section being connected to said third layer via said fourth jumper; said fourth layer being connected to said second lower sheet; said second lower sheet being connected to said second upper sheet by said second plasma switch and said second capacitor bank in series; means for injecting a plasma into said cavity; means for insulating said plasma from said coil whereby discharge of said capacitor banks by said plasma switches causes a varying current to fiow in the surface of said coil; said coil being so arranged and adapted that said current establishes in said cavity a magnetic field having the betatron characteristics; said magnetic field establishing an ion-stabilized, self-focusing, relativistic-electron beam in said cavity from said plasma by induction; plasma gun means for shorting said first and second terminals and second plasma gun means for shorting said third and fourth terminals whereby said electron beam is maintained in said cavity for a relatively long period.

References Cited, in the file of this patent dustries (January 1946) page 83; 

